Reference electrode for ph meters



Y Feb. l2, 1963 VAN DEN BERG 3,977,446

REFERENCE ELECTRODE FORP H METERS Filed June l5. 1959 Saturated with KCI and nonionizinq j soluble glycer'ine 2| INVENTOR L AMBER TUS van den BERG AGENT United States Patent Oiice ddd Patented. Fel). l2, 1963 This invention relates generally to reference electrodes used for the determination of pH of solutions and particularly concerns modified calomel half-cells having utility for measurement of the concentration of hydrogen ions at temperatures lying in the range from about 40 C to about 110 C.

The practice of the invention enables the determination of hydrogen ion concentration of aqueous substances and admixtures to be made over a range of temperatures eX- tending well below the freezing point of water with substantially the same apparatus as has heretofore been employed, and with undiminished accuracy of measurements throughout that range of temperatures in which presently available apparatus is satisfactory. The invention also enables reproducible and substantially accurate determinations of pH to be made for such substances as frozen milldf-oods, and biological matter at their usual`hard frozen storage temperatures, aswell as soils, plant roots and stems, and other natural substances in frozen states.

Apparatus in wide use for measurement of pH presently comprises a cell assembly in which a test electrode, and a reference electrode having a determinable standard potential, are connected by the solution to be tested and the difference of potential developed between the electrodes is equalized by the application of an externally supplied adjustable counter-voltage equal in magnitude to the cell EMF., to provide an indication of the pH. Some of the prior art test electrodes whose differences of potential with respect to a reference electrode in a solution may be related by known functions with the pH thereof, include the quinhydrone electrode, certain pure metal electrodes such as antimony and iridium, the hydrogen electrode, and the ion-conductive glass electrodes. As a reference electrode one-half of a standard Weston cell is used which may take any of various physical forms and which generally comprises a glass tube open at one end containing a quantity of pure mercury in contact with a paste of calomel (HgzClz, i.e. mercurous chloride). A saturated bridge electrolyte solution of potassium chloride in water is placed in contact with the mercury and the calomel paste and affords a conducting path, provision being made for restricted ilow of the electrolyte into the solution being tested, as through a glass tube having a closed end pierced by a capillary bore and dipping into the solution to be tested, or stopped by a porous plug. A platinum body immersed to contact only the mercury provides a means for electrically connecting the reference electrode with the measuring circuit. Such reference electrode has a stable determinable potential against which the variable potential of the test electrode is measured.

The eutectic point of a saturated solution of potassium chloride is 10.7 C. at which temperature solidiication beoins causing the reference electrode assembly to become inoperable. Therefore, while known solid form test electrodes such as the named metal or glass electrodes do,

not of themselves set the limit of the lowest temperature for measurements of pH, the reference electrode cannot be used for theoretical reasons below about 11 C. and in practice it usually fails to give reliablereadings at some temperatures above 11 C. This limitation has handicapped seriously testing and control operations.

I am aware that the substance agar, which is a carbohydrate by nature, consisting mainly of the calcium salt of a sulfuric ester of d-galactose, has heretofore been pro' posed to be added to modify the viscosity of the KCl electrolyte solution to simplify transportationV and handling. However, the eutectic point of a saturated solution of KCl in 1% agar is about 10.6 C. which is essentially the same as the eutectic point of an aqueous potassium chloride solution. Moreover the difference between the potentials Vof a standard calomel electrode and of an agarmodied electrode varies significantly over the practical pH range and introduces error over parts of the range, equiriug additional corrections. These errors are apparently introduced by the adverse effects of the agar on the residual liquid junction potential between the bridge4 solution and the test solution (change of liquid junction potential when the electrode is transferred to a solution of different composition). The presence of agar is also likely to cause an increase in the suspension effect (effect of colloidal particleson the junction potential). lt has been appreciated that both the residual liquid junction potential and the suspension effect may be important errors in pH measurement in any solution :at low temperatures and in biological materials and concentrated solutions at all temperatures.

The present invention essentially consists in the addition to the KCl electrolyte solution in a calomel halfcell, of an organic compound of the class of compounds comprising the stable polyalcohols (polyols), and their stable non-ionizing derivatives which are completely miscible in saturated potassium chloride solutionfin such amount as to substantially depress the freezing point of the resulting solution. Glycerol is a preferred additive, inasmuch as a reference electrode containing equal parts by weight of glycerol and water, and saturated with potassium chloride, can be used down to below 40 C. Without risk of crystallization or solidication of the glycerol or water. Moreover such modied electrolyte solution also has an elevated boiling point enabling measurement to be made of the pH of solutions above 100 C. for example, up to about 116 C.

lt is therefore a primary object of this invention to modify a standard reference electrode by depressing the freezing point of the solution surrounding the electrode, the bridge solution, or both, by admixture therewith of a non-ionizing organic substance which is completely miscible in saturated KCl and which is non-reactive with the cell constituents.

lt is another obg'ect of the invention to provide a modied calomel half-cell reference electrode whose constituents do not solidify at temperatures well below the normal eutectic point of saturated potassium chloride solution,

` and which in addition is appreciablyless affected by errors due to residual liquid junction potentials over a wide range of pH and temperature, and which is substantially less affected by suspended colloidal particles than the standard calomel half-cell electrode.

The practice of the invention in its prefer-red mode may be best understood by reference to the accompanying figures of drawing to which the following description relates, in illustration of modifications of known forms of reference electrodes or half-cells; these figures comprise:

FIG. 1, being an illustration of a reference electrode immersed in a bridge solution having a modifying compound dissolved therein; and

FIG. 2, being a diagram of a known form of reference electrode whose electrolyte constituents include a dissolved modifying compound. i

in a form of apparatus as shown in FIG-1, the bridge solution 'i3 only is modified.` The bridge electrolyte comprises an aqueous solution saturated in potassium'chloride and a selected stable non-ionizing soluble organic comt pound such as glycerol, and is contained in a suitable reservoir `15,-while an unmodified reference electrode l is supported with its lower apertured end immersed in the electrolyte. Unmodilied reference electrode l0 is provided with a connecting lead l1 atits upper end, and has its lower end formed Ywith capillary opening l2 communicating withthe half-cells bridge electrolyte, comprising a saturated aqueous solution of potassium chloride. A reference electrode suitable for the purpose is that designated as type 78026 V calomel reference elec- .trode, commercially available from the Beckman Corporation.

v.ReservoirlS may-contain anyY suitable volume of solution, whichwill be hereinafter designated the secondary bridge electrolyte, inasmuch as this solution is in series -with the half-cells ownor primary bridge electrolyte. `A11 apertured plug lo closes the upper end of the reservoir and supports the reference electrode. A short tubulation 17 extending from the side of the reservoir below the plug serves as a llingarm,iand is suitably capped. The secondary .bridge electrolyte comprises an aqueous solution saturated with KCI and with a modifying organic cornpound such as glycerol, Vthe former being present in amount sucient to provide a crystal bed i4. The reservoir is ported: near itsvlower end.` as by atubulation v125, Awith which an 4elongate conduit@ is connected, hav- -ing a small bore, for example 5 mm. diameter. The other end of the conduit is arranged to have a restricted flow `aperture 21. A stopcock--20, or equivalent shut-olf dehvice, is provided alongthe length of the conduitto enable owrorseepage of secondary bridge electrolyte to be stopped when the half-cell is not in use.

In thenieasurement of pH, a sample 40 of av substance Whose 1hydrogenion concentration is to be determined, and whose temperaturemay be far below 0 C., is placed yin vcontact vwith the tip 2l of the conduit, from which a rninuteowfofthe secondary bridge electrolyte is allowed .toexude A 4test electrode 41 is also brought into contact with the substance, Vto-completean electromotive force ell.. Measurement of the developed between lead V111' and the test electrode is performed in the conventional marmer,- and the net detected may be interpreted most readily by reference to a calibration graph or table including corrections for` `temperatures of each part of the-.GQ11- Theoretically, a temperature gradient alongthe secondtu'y bridge solution would contribute to the net E.M.F., theerror generally increasing with the total temperature difference between .substance 40 and solution t3. The calculationIor measurement of the due to tern` perature difference being extremely difficult, the use of a calibration ltable obviates the problem and such table may bey readily kprepared by one familiar with the art of pH determination. Moreover the Vaddition of a modifying organic Ycompound such as glycerol according to the inventionhas been found bytests not to-affect the magnitudefof the gradient IE.M.-F. as determined by measureinents withreferenceelectrodes not so modified.

".While .only thesecondary bridgerelectrolyte has been describedV as including a modifying compound according to the invention, all-parts'ofa calomel half-cell may have anon-ionizing stablesolubleipolyol such as glycerol added thereto. yReferring to FIG. 2, a reference electrodeit) similar to thatshown in FIG. `l-has .its uid contents admixed `with a modifying compound, permittingthev entire Vhalffcell to be cooled to a low temperature while remaining elfectiveto produce a determinable reference `In the construction of such reference electrodeV a slender glass tube 22cis formed with a narrow necked bulb 23'at its upper end and a llingof mercury 24 placed therein. A conducting lead 11 extends through a press into vthe half-cell and is joined with platinum wire 25 in contact with the mercury. The remainder of the tube 22 is packedA with asemi-solid aqueous paste 26 whose active Aingredients are mercurous chloride (HgzClz), and

KCl. A stable non-ionizing, soluble polyol or a suitable derivative according to the invention is admixed thoroughly with the paste in amount sucient to modify the freezing point. To prevent settling, the paste is loaded with a large volume percentage of an inert solid material such as asbestos, glass particles, or the like. An aperture 27 is formed in the side of tube 22 at its lower end to expose the paste in contact with the bridge electrolyte i3 surrounding its lower portion. A coaxially disposed tube l0 of larger diameter having its upper end sealed by cap 30 serves as a reservoir and extends below tube 2,2, tapering to a point, which is provided with a capillary bore l2. The bridge electrolyte is composed of water, an excess of potassium chloride, and a modifying organic compound, preferably the same as that admixed with paste 26.

The group of compounds useful as additives in the half-cell or in the bridge solution in contact with a solution to be tested, may generally be stated to comprise all water soluble polyaloohols, and in particular those which are completely soluble in saturated KCl solution in the desired temperaturev range. Polyalcohols can generally be described by the formula lwherein n is any whole number greater than '1, and R1 tand R2 are radicals, for exampleH, CH3, 02H5. In addition the stable, non-ionizing derivatives of thestable polyalcohols are also edective provided that they are rcompletely miscible with saturated KCl solution inthe `desired temperature range. Such condensation products may generally con'rprise ethersof apolyalcohol:

Ra-o-R,

wherein Rsis ar polyalcohol having one OH-group ref moved from its molecule. Since R3 always has more .than two OH-groups,fother molecules of the polyalcohol may `be linked tothe groups thereby to form larger molecule structures. Ethers ofpolyalcohols can also be represented by the formula 11a-oma,

wherein R4 is an alcohol other than R3. Condensation products may further include compounds wherein R3 itself isa condensation product of a polyalcohol.

In illustration of the foregoing, the relatively nonviscous, non-ionizing completely soluble and non-volatile :stable polyalcohols and their derivatives useful in the realization of the invention include glycerol, ethylene glycol, diethylene glycol, and diethylene glycolmono-ethyl ether. The foregoing compounds are substantially noncreactive wi'th the calomel half-cell constituents or with potassium chloride.

In a specific example, the addition of glycerol in proportions previously stated enables the cellor bridge solution to be cooled as low as 40 C. without risking crystallization. potential difference determinations made with such an electrode and a suitable low temperature glass electrode differed less than 2 mv. from read-ings made with an unmodified saturated calomel electrode in dilute solutions between pH l and pH 10, at temperatures between -1-0to +40 C. as measured with a Beckman G pH meter.

'Ifhe residual liquid junction potential measured between 0.01 N and 1.0 N hydrochloric acid at 25 C. was 11 mv. for the unmodified calomel electrode, but only 5 mv. for the modified electrode. The suspension elect measured with thermodiied electrode in soils varied between 0 and +4 mv. while the suspension eect measured with the unmodified electrode Varied between -4 and mv. Measurements with suspensions of ion exchange resins showed that the addition of glycerol to the electrode and bridge solutions produced a considerable decrease in the suspension effect.

` lt will be apparent from the foregoing that the addition of glycerol to the electrode and bridge solutions of calornel half-cell electrodes modifies the reference electrede to render it useful over a wider temperature range than has hitherto been possible, and achieves both a smaller residual liquid junction potential and a smaller error due to suspension effect. Table I is given in illustration of the residual liquid junction potential in 1.0 N HCl and in 1.0 N NaCl-l solution at room temperature. It will be apparent that with increasing weight proportion of each organic compound with respect to water, the mol/liter concentration of potassium chloride decreases. Depending on `the application and the lowest temperature limit at which measurements areto be made, any system may be adjusted by choosing the appropriate percentage weight of the modifying compound. ln general, equal parts by weight of modifying substance and water extend the operating range of the half-cell to temperatures below `about -30 C. or lower.

While liquid modifying compounds of the class described are generally preferred. as yielding modified electnolytes of low viscosity, it is also contemplated that relatively viscous solutions of non-ionizing polyols and their derivatives may be provided for the half-cell electrolyte, the primary bridge electrolyte or the secondary bridge electrolyte for the purpose of rendering the apparatus more readily movable.

Although the foregoing description has been restricted to a discussion of calomel reference electrodes, it is to be understood that other half-cells having determinable values, such as the half-cell comprised yof a pure silver electrode in co-ntact with :a saturated electrolyte of AgCl and KCl may be similarly modified by an organic compound such as glycerol according to the invention, provided that the compound selected is nonreactive with any of the cell constituents over a useful range of pH of test solutions.

Table I RESIDUAL LIQUID JUNCTION POTENTIALS (AT 25 C.) OF

OF AQUEOUS SOLUTIONS SATURATED VITH KCl AND CONTAININC- .TMODIFYING ORGANIC COMPOUNDS l claim:

l. A reference half-cell comprising a tubular vessel having a capillary opening at one end for flow of liquid therethrough, a reference electrode supported in said vessel, and an electrolyte liquid disposed in said vessel in contact between said opening and said reference electrode,

said electrolyte liquid consisting of water, potassiumy chloride, and a non-ionizing soluble organic compound admixed therewith in amount sufficient to depress the freezing point of said electrolyte below -l0.7 C., said compound being selected from the class of substances consisting of glycerol, ethylene glycol, diethylene glycol, and diethylene glycol mono-ethyl ether.

2. A reference half-cell as in claim l wherein said reference electrode comprises the system:

(mercuryrmercurous chloride:bridge electrolyte) where said bridge electrolyte comprises a saturated solution of potassium chloride in water.

3. A reference half-cell as in claim 1 wherein said reference electrode comprises the system:

(silver:silver chloride :bridge electrolyte) where said bridge electrolyte comprises a saturated solution of potassium chloride in water.

4. A reference half-cell as in claim 2 wherein said bridge electrolyte further includes a quantity of said organic compound sufficient to depress the freezing point of the bridge electrolyte below 10.7 C.

5. A half-cell for use in determination of concentration of hydrogen ions in a substance having an ion-containing liquid portion, comprising a calomel reference electrode having a primary electrolyte saturated in potassium chloride in contact with said calornel electrode and mercurous chloride, and a secondary electrolyte in contact with said primary electrolyte adapted to contact said substance, said secondary electrolyte consisting of an admixture in water of an excess of potassium chloride and yan amount of a stable soluble non-ionizing organic compound suflicient to depress the freezing point of said secondary electrolyte below 10.7 C., said compound being selected from the class of compounds consisting of glycerol, ethylene glycol, diethylene glycol and diethylene glycol mono-ethyl ether.

6. A half-cell for use in determination 0f concentration of hydrogen ions in a substance having an ion-containing liquidportion, comprising a silver-silver chloride reference electrode having a primary electrolyte saturated in potassium chloride andsilver chloride in contact with said silver-silver chloride electrode, and a secondary electrolyte in contact with said primary electrolyte adapted to contact said substance, said secondary electrolyte consisting of an admixture in water of an excess of potassium chloride and an amount of a stable soluble non-ionizing organic compound sufiicient to depress the freezing point of said secondary electrolyte below 10.7 C., said compound being selected from the class of substances consisting of glycerol, ethylene glycol, diethylene glycol and diethylene glycol mono-ethyl ether.4

7. A reference electrode for the measurement of the pH of substances having an ionized liquid constituent over a range of temperatures extending from below the freezing point of a eutectic mixture of KCl and Water to above the boiling point of said eutectic solution, comprising a tubular vessel of insulating material having one end closed, a mercury electrode in said closed end, and aqueous electrolyte saturated with mercurous chloride and potassium chloride filling said Vessel in Contact with the mercury, said electrolyte having admixed therewith a soluble prganic compound seleted from the group vconsisting of glyceroLethylene glycol, diethylene glycol, and diethylene glycol mono-ethyl ether, in an amount ranging from about 20% to about 100% by weight of the water constituent of saidelectrolyte.

S. A reference electrode for the measurement of the pH of substances having an ionized liquid constituent over a range of temperatures extending from below the freezing point of .a eutectic mixture of KCl and water, to above the boiling point of said eutectic solution, comprising a tubular vessel of insulating material having one end closed, a silver electrode in said closed end, and aqueous electrolyte saturated with silver chloride and potassium chloride filling said vessel in contact with the silver, said d electrolyte having admixed therewith a soluble organic compound selected fromthe group consisting of glycerol, ethylene glycol, diethylene glycol, and diethylene glycol mono-ethyl ether, in an amount ranging from about 20% to about 100% by weight of the water constituent of said electrolyte.

9. A reference half-cell as in claim 3 wherein said bridge electrolyte further includes a quantity of said organic compound sufficient to `depress the freezing point of the bridge electrolyte below 10.7 C.

References 'Cited in the le of this patent UNITED STATES PATENTS Handforth Oct. 21, 1930 Parker et al Feb. 23, 1932 Wilson Oct. 23,1945

Burgess Dec. 23, 1947 Beckman et al.V July 17, 1956 FOREIGN PATENTS Great'Bi-itain 1883 

1. A REFERENCE HALF-CELL COMPRISING A TUBULAR VESSEL HAVING A CAPILLARY OPENING AT ONE END FOR FLOW OF LIQUID THERETHROUGH, A REFERENCE ELECTRODE SUPPORTED IN SAID VESSEL, AND AN ELECTROLYTE LIQUID DISPOSED IN SAID VESSEL IN CONTACT BETWEEN SAID OPENING AND SAID REFERENCE ELECTRODE, SAID ELECTROLYTE LIQUID CONSISTING OF WATER, POTASSIUM CHLORIDE, AND A NON-IONIZING SOLUBLE ORGANIC COMPOUND ADMIXED THEREWITH IN AMOUNT SUFFICIENT TO DEPASS THE FREEZING POINT OF SAID ELECTROLYTE BELOW -10.7*C., SAID COMPOUND BEING SELECTED FROM THE CLASS OF SUBSTANCES CONSISTING OF GLYCEROL, ETHYLENE GLYCOL, DIETHYLENE GLYCOL, MAINTAINED AT A TEMPERATURE NOT SUBSTANTIALLY ABOVE ABOUT 500*F., IN SAID FIRST SEPARATION ZONE SEPARATING THE FIRST PRODUCT EFFUENT AND RECOVERING THEREFROM A FIRST HYDROGEN-RICH GASEOUS STREAM AND A FIRST LIQUID HYDROCARBON PRODUCT STREAM, PASSING THE THUS SEPARATED HYDROGEN:RICH GASEOUS MATERIAL WITH A SECOND HYDROCARBON FEED MATERIAL IN CONTACT WITH A DESULFURIZATION CATALYST IN A SECOND DESULFURIZATION ZONE MAINTAINED UNDER ELEVATED TEMPERATURE AND PRESSURE CONDITIONS, RECOVERING A SECOND PRODUCT EFFLUENT FROM SAID SECOND DESULFURIZATION ZONE, COOLING SAID SECOND PRODUCT EFFLUENT AND SEPARARATING INTO A SECOND HYDROGEN-RICH GASEOUS STREAM A SECOND LIQUID HYDROCARBON PRODUCT STREAM, RECOVERING AND TREATING SAID SECOND HYDROGEN-RICH GASEOUS STREAM TO REMOVE HYDROGEN SULFIDE THEREFROM AND RECYCLING A PORTION OF THE THUS TREATED SECOND HYDROGEN-RICH GASEOUS STREAM TO EACH OF SAID DESULFURIZATION ZONES, PASSING ANOTHER PORTION OF SAID TREATED SECOND HYDROGEN-RICH GASEOUS STREAM WITH SAID FIRST LIQUID HYDROCARBON PRODUCT STREAM TO AN ADDITIONAL SEPARATION ZONE TO RECCOVER A RELATIVELY LOW-BOILING HYDROCARBON FRACTION FROM A THIRD LIQUID HYDROCARBON PRODUCT, RECOVERING SAID THIRD LIQUID HYDROCARBON PRODUCT AND PASSING THE THUS RECOVERED RELATIVELY LOW-BOILING HYDROCARBON FRACTION TO SAID SECOND DESULFURIZATION ZONE. 